1. Field of the Invention
This invention relates generally to a microminiature integrated circuit adjustable impedance network and more particularly, to a microminiature integrated circuit resistance network comprising a plurality of weighted resistance elements which may be selectively interconnected to provide a predetermined fixed resistance, or resistance ratio.
2. Description of the Prior Art
For many years military and industrial reliability specialists have been dissatisfied with the vulnerability of sliding contact adjustable resistors and trimming potentiometers to variation after the impedance has been adjusted to a predetermined value. Generally the impedance of trimming potentiometers built into electronic equipment is adjusted to the desired value with a single one-time adjustment either at the equipment factory or at a field installation of the equipment. However, variations have occurred regardless of where the adjustment is made and whether the sliding contact potentiometer is of the wire-wound or of the nonwire-wound types. Nonwire-wound potentiometers are typically fabricated from materials such as cermet, carbon film, hot-molded carbon, thin film metal and conductive plastic.
Adjustment is especially affected vibration, temperature cycling, and chemical corrosion. For example, under vibration conditions, it has been found that the variation of the contact position on a potentiometer may cause an end-to-contact slider resistance that changes by as much as 3 percent of the end-to-end resistance of the potentiometer. It should also be recognized that the resolution of wire-wound potentiometers inherently, in addition to contact variation, changes in steps.
In the prior art, trimming potentiometers are available to provide linearly adjustable resistances in a miniature package. Typically, several materials are used to fabricate the trimming potentiometers, with the most common kinds of materials including resistance wire, cermet thick film, hot-molded carbon and carbon film. In general, the wire-wound types are more stable with temperature variation and have a lower contact resistance than the thick film elements. As previously mentioned, wire-wound potentiometers have an inherent resolution disadvantage because of the step resistance changes which occur as the sliding contact, or wiper, wipes from one turn to the next, a disadvantage which does not occur with the continuous nonwire-wound type. However, the "infinite" ressolution associated with the continuous elements is often of little real value because of the fact that in all miniature trimming potentiometers, the contact setting can change after its initial setting by significant amounts, especially when the potentiometer is exposed to shock, vibration, and chemical corrosion.
Some of the key parameters of the several types of trimming potentiometers have been investigated. With respect to these parameters it has been found that a wire-wound trimming potentiometer having an end-to-end resistance of about 10 ohms has a manufacturing tolerance of approximately .+-.20%, an end-to-slider resistance resolution of about 1%, end-to-slider resistance variation after vibration of about 2%, and a slider potentiometer voltage variation after vibration of about 3%; that a wire-wound trimming potentiometer having a full scale end-to-end resistance of about 100K ohms has a manufacturing tolerance of .+-.5%, an end-to-slider resistance setting resolution of about 0.1%, an end-to-slider resistance variation after vibration of about 0.5%, and a slider potentiometer voltage variation after vibration of about 0.5%. In contrast, a cermet potentiometer having a resistance of about 10 ohms has a manufacturing tolerance of about .+-.20%, an end-to-slider resistance setting resolution of about 0.05%, an end-to-slider variation after vibration of about 3%, and a slider potentiometer voltage variation after vibration of about 3%.
Some of the particular disadvantages of the principal types of trimming potentiometers are as follows: a wire-wound trimming potentiometer has step resolution, contact noise, contact resistance which increases with corrosion and load life, a poor high-frequency response, a relatively poor reliability, a substantial material cost, a high adjustment cost due to the labor involved and exhibits contact shift under vibration and shock; and a cermet trimming potentiometer has a greater contact noise and contact resistance than the wire-wound potentiometer, and similar to the wire-wound trimming potentiometer, exhibits contact shift under shock and vibration, has a relatively poor reliability, and has a high material and adjustment cost.
In some applications a trimming potentiometer is required to have a resistance which controls a parameter such as current which may vary such that its value is proportional to the inverse of the resistance. To vary the current linearly with setting, the reciprocal of potentiometer resistance would have to vary "linearly" with slider setting, thus manifesting a hyperbolic relation between resistance and sliding setting. Such trimming potentiometers are not commercially available, and are not feasible by conventional slider techniques. In such applications, along with the disadvantages described previously, the linearly adjustable potentiometer has the additional disadvantage that the reciprocal of the resistance becomes very nonlinear at low values of resistance.
In other applications trimming potentiometers may be connected as a continuously adjustable attenuator. In attenuator applications, all potentiometers, including wire-wound trimming potentiometers and film trimming potentiometers are disadvantageous because of their wide variation of output resistance, whereas wire-wound precision potentiometers have the added disadvantage of their expense, poor high frequency response and size.
Weighted resistor networks have been commonly utilized in D/A and A/D converters during the past 20 years to obtain a precise value of resistance or resistance ratio. Examples of discrete element weighted resistor networks used or usable in digital-to-analog converters are found in "Notes on A/D Conversion Techniques," written by A. K. Susskind, and published in 1957 by MIT Press, and in U.S. Pat. Nos. 3,723,143, Wasserman, and 2,892,147, Bell. But such networks, which have been utilized for digital control and readout of voltage, current, and resistance in instrument and control applications, have utilized high accuracy resistance elements and switches which are too large and costly to compete effectively with electromechanical trimming potentiometers. Moreover, besides the deficiencies of size and cost when considered for use in trimming potentiometer applications, these weighted D/A networks present the added difficulty that the only method available to insert them into a system and adjust them to a value of impedance satisfying the system requirements has necessitated nonvolatile-memory-containing switches which must be resettable during adjustment, yet hold setting with power off, such as latching relays, which are relatively large, expensive and unreliable, with more contacts than the trimming potentiometers.